The present invention relates to a method of forming a metal article, such as an airfoil, from powdered metal.
A method of forming an article, such as a turbine blade, of metal powder is disclosed in European Patent Publication No. 0 172 658 A1, published Feb. 26, 1986 from application Ser. No. 85305176.1 filed July 19, 1985 and entitled Method of Forming Powdered Metal Articles. This publication discloses forming a container by electroplating a layer of metal over a pattern. The pattern is subsequently removed from within the layer of metal to leave the empty container which is filled with a metal powder. After the container has been filled and sealed, the container is subjected to a hot isostatic pressing operation which results in a bonding of the particles of metal powder to form a unitary body.
A method of making a gas turbine blade or rotor from powdered metal is also disclosed in U.S. Pat. No. 4,329,175 issued May 11, 1982 and entitled Products Made by Powder Metallurgy and a Method Therefor. This patent teaches that the physical characteristics of an article formed of metal powder can be made different in different parts of the article by using different metal powders to form different parts of the article. Thus, a first portion of a container is filled with a standard metal powder and a second portion of the container is filled with a treated metal powder having a different physical characteristic.
The treated metal powder referred to in U.S. Pat. No. 4,329,175 is formed by conducting the standard metal powder between a pair of rolls. The rolls mechanically deform the powder in a manner similar to that disclosed in U.S. Pat. No. 3,976,482 issued Aug. 24, 1976 and entitled Method of Making Prealloyed Thermoplastic Powder and Consolidated Article. By using plastically deformed metal powder particles to form one portion of the blade and standard or undeformed particles to form another portion of the blade, different grain growth characteristics are obtained when the metal powder is subjected to a hot isostatic pressing operation.
When metal powder is to be deformed by passing between a pair of rolls, in a manner similar to that taught by the aforementioned U.S. Pat. No. 3,976,482, powder particles are first separated into different sizes and then passed through an appropriately spaced gap in a rolling mill. This results in plastic deformation or strain energizing of the metal powder particles. However, with this process, it is probable that under-sized particles will pass through the rolling-mill gap and will not be cold worked. The particles which are not cold worked will not subsequently recrystallize to a relatively fine grain size. The coarser microstructure region resulting from the powder particles which were not cold worked provides a site for the initiation of a fatigue crack.